1. Field of the Invention
The present invention relates to a surface processing technique of a substate which is processed to fabricate a semiconductor device. Particularly, this invention relates to a surface processing technique for removing or dry etching of an oxide film formed on said substrate.
2. Description of the Prior Art
It has been found that a native oxide film formed on the surface of a semiconductor substrate such as silicon, and on a semiconductor or metal film formed on said substrate, causes a problem in the fabrication of a semiconductor device from that substrate. This film is formed easily by exposing the substrate to the air, or delivering the substrate between fabrication apparatus. This film is usually very thin (for example, the film thickness of silicon native oxide is about 5-20.ANG.-that is, oxide having 2-3 atomic layers), but this causes difficulties in the fabrication processes and affects the electrical properties of semiconductor devices.
For example, it is significant in case that a layer of one material formed on a semiconductor substrate is etched selectively, leaving layers of other materials, such as Si selective etching to silicon dioxide. It is also significant in a selective CVD process or the like in which a film should be deposited selectively on a particular part of a substrate, surface such as contact holes or through holes.
The selective etching and the selective CVD process mentioned above may be done utilizing the differences in properties between materials. Consider a case in which semiconductor materials such as silicon; metals such as aluminum; and their nitrides are etched by electrically neutral atoms or molecules. In this case, aluminum and silicon can be etched by chlorine gas, and silicon nitride can be etched by a interhalogen gas such as clF; but the oxides cannot be etched by these gases at all. A selective etching is therefore possible between these materials.
When a native oxide film is formed on the surface of a semiconductor or a metal, however, the selectivity between these materials is lost somewhat, whereby poor etching is caused.
Recently, a new method for better selective etching has been developed. In this method, a reactive ion etching apparatus equipped with means of making a magnetron plasma and cooling the substrate is used. Consider a case in which a silicon substrate is to be etched by chlorine gas, leaving a silicon dioxide film using the method mentioned above. SiCl.sub.4 is produced as a silicon etching product and SiCl.sub.x (x=1-3) is produced from the X decomposition of SiCl.sub.4 in the magnetron plasma. These solar materials are easily adsorbed on a silicon dioxide surface which has polarity and as a result, an adsorbed layer is formed. The surface of the silicon dioxide cannot be etched, because the adsorption layer prevents etching. On the other hand, a silicon substrate can be etched, because it has no polarity and an adsorption layer to prevent etching is not formed. As a result, selective etching can be successfully carried out in this case. The etching selectivity becomes worse, however when a native oxide film is formed on a silicon substrate surface.
In a case of selective CVD using WF.sub.6, H.sub.2 and silane gases or the like, tungsten is deposited on a silicon or metal surface of the substrate by a reaction between said surface materials and said gases if the substrate is kept at a suitable temperature. On the other hand, tungsten cannot be deposited on a surface covered with oxides of said substrate materials, because said gases cannot react with these oxides. Thus, a selective CVD is executed between silicon, metal and their oxides. In this case, the etching selectivity is reduced, and so an efficient selective CVD cannot be performed if the silicon or metal is covered with a natitive oxide film.
In cases of molding wires and electrode materials into contact holes or through holes, the resistivity of these holes increases when the substrate is covered with a native oxide film. As a result, the operating speed of devices made of this substrate decreases.
The deterioration of etching selectivity mentioned above is particularly significant in processes in which a chemical reaction is dominant. It is also significant in processes such as the reactive ion etching process, in which a physical interaction caused by an irradiation of energetic ions is added to the chemical reaction.
As mentioned above, good selectivity for etching or deposition cannot be obtained when a surface to be processed is covered with native oxides. Currently no treatments for removing the native oxides have been successfully developed. With the scaling down of electronic devices, however, it is highly desired to obtain a reliable technique for removing the native oxides easily, in order to carry out good selective etching or deposition.
The native oxide formed on a substrate causes problems not only in etching and CVD but also in the processes of oxidation, epitaxial growth and diffusion.
In a formation of an oxide film (especially gating oxide film), it is highly desirable that the film formed should have a high quality, and uniform thickness. However, if a native oxide film exists on a substrate before the formation of said oxide film by high temperature oxidation, new oxide film grows up over the poor quality native oxide film, which is formed at room temperature. The quality of the gating oxide is lowered, as a result. Furthermore, the thickness of the gating oxide becomes nonuniform, because the thickness of the native oxide formed before the formation of said gating oxide is not uniform, and has no reproducibility in thickness.
In an epitaxial growth on a silicon substrate, the surface of the substrate should be very clean and native oxide must be removed in order to obtain a silicon epitaxial film of good quality. In an actual process, a substrate surface having native oxide is first treated by wet cleaning to remove organic and metal contaminants or the like from the surface of the substrate. Then, the substrate is placed in a vacuum chamber for epitaxial growth. HCl or H.sub.2 gas is then introduced into the chamber, and the substrate is heated in order to change the native oxides into SiO. The SiO film is removed easily by sublimation from the heated substrate. Contaminants on walls of the chamber break away from the walls, however, and adhere to the substrate during said heating process.
In the vapor or solid state diffusion process of B, As and P into a silicon substrate, the diffusion efficiency may also be reduced if a native oxide is formed on the substrate because it becomes a diffusion barrier.
Other than the native oxides mentioned above, it is also necessary to remove or etch an oxide film formed by the process of fabricating a semiconductor device.
In a DRAM fabrication process, a technique is used in the formation of a so-called trench capacitor. In this technique, a capacitor is made on an inner surface of a trench formed in a silicon substrate in order that it occupies a small area without decreasing the capacity of this area. But if the corners of the trench is steep, the electric field of the gating oxide at the corners becomes very high, and dielectric breakdown can be easily occurred. To avoid this, the substrate should be processed as follows. First, the substrate having trench is oxidized at a high temperature above 900.degree. C., so as to form a uniform oxide film on the trench. This oxidation process is called `sacrificing oxidation`. Removing this oxide film, a trench with rounded corners and smoothed surface is gained. Then, a gating oxide film is formed on the trench surface of the substrate. The corners of the trench is rounded, so that the insulation breakdown can be avoided.
In this process, the removal of said oxide film should be done without damaging the silicon substrate, and it should only be selectively removed from the substrate.
Furthermore, the substrate having trench may follow CVD process to deposit impurity-containing silicon dioxide on the trench surface, to form electrodes inside the trench. The impurities in the deposited oxide film will be diffused into the silicon substrate by heating. After this diffusion, the silicon dioxide film should be removed selectively, without damaging the underlying substrate.
When forming contact holes or through holes by etching a part of the silicon dioxide film formed on the silicon or wire, the etching should be carried out without damaging the underlying materials. Such damage increase the contact resistivity of these holes.
If the substrate is repeatedly processed in a reaction chamber, silicon dioxide or the like sometimes adheres to the back surface of the substrate and the surface of the chamber walls. This occurs, for example, when a deposition gas containing silicon or the like is introduced into a reaction chamber. In this case, the oxides adhering to the chamber wall peel off and deposit on the substrate so as to lower the yield of devices made of the substrate. It is highly desirable, therefore, that the oxide deposited on the substrate be easily removed before it is subjected to the next process.
Furthermore, the oxide film mentioned above sometimes contains alkaline metals or heavy metal and should be removed from the substrate. Usually the removal of said film from a substrate is done by wet etching, using hydrofluoric acid or a hydrofluoric acid/ammonium fluoride buffer solution. In this method, the oxide can be selectively removed from the silicon substrate, without damaging the underlying substrate.
The oxide formed inside a trench which has a high aspect ratio (trench depth vs. apparture radius of the trench), however, cannot be easily removed by the above mentioned wet etching method, because surface tension makes it difficult for an etchant to enter the trench. Furthermore, wet etching produces silicates on substrates as a result of the etching reaction. These silicates should be completely removed by a rinse using pure water. If the rinsing is not done through, however, the silicates remain on the substrate surface to form colloid and spots on the surface. It is also not easy to deal with hydrofluoric acid and hydrofluoric acid/ammonium buffer solution because they have strong toxicity. Furthermore, a native oxide film will again be formed on the substrate when it is exposed to the air after the wet etching.
To solve the problem of using the wet etching mentioned above, a dry etching method has been proposed, which uses gas to remove the oxides. Oxides of silicon or the like can be dry-etched by fluorine atoms for example.
Fluorine atoms etch silicon and metal substrates along with silicon dioxides, however, therefore selective etching of silicone dioxide alone cannot be achieved by this method.